People have been heating and cooking food for a long time and, over the centuries, have created a variety of devices for heating food, beginning with an open fire, and moving on to a stove, an oven, a microwave oven, a griddle, a fryer, and so forth. Almost all of the heating devices heat to a high temperature, well above the boiling point of water. These devices can be very useful for cooking the food if the cook is attending closely to be sure the food does not burn or dry out during the cooking process, and if the food is to be eaten promptly after it is cooked. However, these devices are not good for re-thermalizing and holding frozen foods or other pre-cooked foods at a ready state for any extended period of time without causing deterioration of the food. Also, because these devices must be able to heat to a high temperature, they use large amounts of energy, so that, for example, an electric stove must be connected to 220 volt power rather than the usual 110 volt household current.
There are several devices that have been constructed for holding food, but they all involve the use of temperatures at or above the boiling point of water. Food is mostly made up of water, so, at those temperatures, the vapor pressure inside the food increases, driving water out of the food, and thereby causing the food to dry out and deteriorate as it is held.
For example, it is possible to purchase pre-cooked, frozen chicken breasts, but there is currently no good, practical way, with the existing technology, to rethermalize those chicken breasts and hold them at a safe temperature, in which pathogens would be killed, without also drying out the meat. If the chicken breasts are put into an oven, they dry out. If they are put into a microwave, they dry out, and a microwave is not designed to hold food, only to heat it quickly. If they are heated on top of the stove or in an electric griddle or frying pan, again the high temperatures cause them to dry out.
U.S. Pat. No. 5,701,804 "Liebermann", which is hereby incorporated by reference, describes a clamshell grill with upper and lower plates that are heated by a circulating fluid to a set temperature, which is below the boiling point of water but high enough to kill pathogens. This device solves the problem of how to thermalize and hold food without causing the food to deteriorate, but it is a very expensive device to manufacture and maintain, and it occupies a large volume of space for a relatively small food contact surface area. The Liebermann patent teaches that the way to thermalize the food and maintain the food at the desired temperature is to provide a reservoir of fluid at a fixed temperature between 130.degree. F. and 185.degree. F., prepare special upper and lower hinged food contacting plates with a fluid pathway in each plate, press the food between the plates, and continuously pump fixed temperature fluid through the plates to maintain the plates at a fixed temperature.
As one might imagine, this fluid-heated grill is a large, heavy, bulky device. The surface on which the food rests is at counter-top height, and the large, hinged lid opens upwardly. The fluid reservoir, heater, and pump are housed below the surface. There are three cooking surfaces per device, so the device occupies a floor or countertop area approximately equal to one-third of the total cooking surface area and occupies a height of at least three feet, or a volume of at least one cubic foot per square foot of cooking surface area. The special plates with fluid pathways are expensive to manufacture and are heavy. Since the food is pressed between upper and lower plates, only one thickness of food can be heated between the plates at any given time.
Mr. Liebermann selected a heat transfer fluid rather than electrical resistance heating in the design taught in that patent, because he knew there were problems with hot spots in electrical heating devices, and hot spots would not be acceptable in a design that required a fixed temperature to prevent the deterioration or drying out of the food.
In a typical electric griddle or frying pan, a metal plate is heated by electrical resistance heating. There is a control that allows the cook to set a temperature from a relatively low warm or simmer temperature to a high temperature on the order of 400.degree. F. In order to achieve the high temperature, the griddle or frying pan must be designed to draw a large amount of power when it is on. That means that, when it is set at a low temperature, it is actually cycling on and off, making large temperature swings on the order of .+-.20.degree. F. Also, because of the high power draw of the resistance heater, the heater becomes hot very quickly, creating hot spots on the griddle or frying pan. Thus, the typical electric griddle or frying pan would not be suitable for heating and holding food in a narrow temperature range below the boiling point.
U.S. Pat. No. 5,403,997 "Wimpee" recognizes the problem of hot spots with electrical heating systems and suggests the use of two different sets of resistance heaters at different power levels, so that the control system would cycle between the two sets of resistance heaters rather than cycling on and off. However, even if the Wimpee system eliminates the problem of hot spots, it still teaches that a food warmer should operate above the boiling point, in the range of 215.degree. F.-240.degree. F., which causes deterioration of the food, and its proposed system for eliminating hot spots would be complicated and expensive.